Navigation by the use of dead reckoning requires very accurate knowledge of velocity and heading as inputs for the navigation computer. A large number of military aircraft use Doppler navigation radar to provide the true ground velocity measurement for this purpose. The use of Doppler radar could be extended to a broader spectrum of aircraft types if its capability and/or cost could be improved.
For example, very high performance, long range aircraft would benefit from improved accuracy and better operation over water. At the other end of the aircraft spectrum, low performance aircraft could use a lower cost Doppler radar.
Operation over water poses a twin difficulty for current Doppler radar ground speed indications. When the water surface is very smooth, loss of signal is experienced forcing the Doppler radar navigator into memory. For other sea conditions, Doppler systems experience a change in groundspeed calibration constant unless very narrow beams are used. The search for solutions to these problems has led to improvements in Doppler radars with respect to the calibration shift over water. It has also led to consideration of alternatives to Doppler radar such as Microwave Interference Pattern Sensor (MIPS) which, by way of contrast, functions well over smooth water. Since its beams are pointed straight down, operations over water result in an increase in signal level relative to operation over land or rough water. Furthermore, the capability of reduced over water calibration shift is inherent to the MIPS technique. This improved operation over water is a real benefit offered by MIPS relative to conventional Doppler radar navigators.
In my U.S. Pat. No. 3,838,424 and U.S. Pat. No. 3,974,500 assigned to the same assignee as the present invention, there is described a MIPS velocity sensing technique which makes use of the speckled nature of the power backscattered from the ground when it is illuminated by a monochromatic radar beam. Translation of the source of the radar beam; i.e., the transmitting antenna, causes the entire speckle pattern to move as a whole in the opposite direction at the same speed. A pair of receiving horns which lie essentially along the line of motion will sense essentially the same power variations except for a delay. With a fixed horn separation, any measure of delay is also a measure of vehicle velocity.
As disclosed in my patents cited above, a previous two axis MIPS used a single large transmitting horn and three small receiving horns. Its speckles were on the average circular and had no preferred orientation. Hence, for that speckle pattern arrangement, the effective distance between any two receiving horns was the actual separation projected onto the velocity vector. As a result, a fairly cumbersome computer was required to generate two orthogonal velocity components and a considerably smaller drift angle could be tolerated for a specific pairing arrangement among the three receiving horns.